Invoking Circuit Switched Fallback in Response to VoIP Call Setup Failure

1. In a communication system comprising a first radio access network (RAN) configured to serve user equipment devices (UEs) according to a first air interface protocol, a second RAN configured to serve UEs according to a second air interface protocol, and a network node through which voice over Internet Protocol (VoIP) signaling packets normally flow between the first RAN and a VoIP call server on a packet-switched network, a method comprising: detecting by the node (i) that a UE served by the first RAN is attempting VoIP call setup to a call destination, and (ii) failure of the VoIP call setup, wherein detecting that the UE is attempting the VoIP call setup and failure of the VoIP call setup comprises receiving a request to establish a dedicated VoIP bearer for the UE and detecting failure to set up the dedicated VoIP bearer, wherein the node is a gateway that provides connectivity between the first RAN and the packet-switched network, and wherein the dedicated VoIP bearer is between the UE and the gateway; and responsive to the detecting, signaling by the node to invoke setup of an alternate call between the UE and the call destination via the second RAN.

2. The method of claim 1 , wherein the alternate call is a circuit-switched-fallback (CSFB) call, and wherein the UE transitions from being served by the first RAN to being served by the second RAN to engage in the CSFB call.

3. The method of claim 1 , wherein the first air interface protocol is Long Term Evolution (LTE) and the second air interface protocol comprises a protocol selected from the group consisting of Code Division Multiple Access (CDMA) and Global System for Mobile Communication (GSM).

4. The method of claim 1 , wherein detecting that the UE is attempting the VoIP call setup and failure of the VoIP call setup comprises: receiving packet data associated with of the VoIP call setup and determining based on the packet data that the VoIP call setup has failed or will fail.

5. The method of claim 1 , wherein the first RAN includes a mobility management entity (MME), and wherein the signaling by the node to invoke setup of the alternate call between the UE and the destination via the second RAN comprises signaling that results in the MME invoking setup of the alternate call.

6. The method of claim 5 , the second RAN includes a switch providing connectivity with a public switched telephone network (PSTN), and the MME is communicatively linked with the switch via an interworking server (IWS), and wherein the MME invoking setup of the alternate call comprises signaling from the MME to the switch via the IWS to cause the switch to (i) originate the alternate call between the UE and the destination via the PSTN, (ii) page the UE to cause the UE to transition to be served by the second RAN, and (iii) connect the originated alternate call with the UE via the second RAN.

7. The method of claim 5 , wherein the UE is served by a base station of the first RAN, and wherein the MME invoking setup of the alternate call comprises signaling from the MME to the base station and wirelessly from the base station to the UE to cause the UE to (i) signal through the first RAN to the second RAN to cause the second RAN to set up the alternate call and (ii) transition to be served by the second RAN to engage in the alternate call.

8. A method comprising: at a gateway that provides connectivity between a Long Term Evolution (LTE) access network and a packet-switched network, receiving packet-data associated with an attempt by a UE served by the LTE access network to set up a voice over Internet Protocol (VoIP) call to a call destination; detecting by the gateway failure of the VoIP call setup, wherein detecting failure of the VoIP call setup comprises detecting failure to set up a dedicated VoIP bearer for the UE, wherein the dedicated VoIP bearer is between the UE and the gateway; and responsive to the detecting failure of the VoIP call setup, signaling from the gateway to a mobility management entity (MME) of the LTE access network to invoke setup of a circuit-switched-fallback (CSFB) call between the UE and the call destination in place of the VoIP call.

9. The method of claim 8 , wherein detecting failure of the VoIP call setup comprises reading the received packet-data and determining that the received packet-data contains an error code indicating failure of the VoIP call setup.

10. The method of claim 8 , wherein signaling from the gateway to the MME to invoke setup of the CSFB call in place of the VoIP call comprises signaling from the gateway to the MME to cause the MME to signal to a fallback access network to cause the fallback access network to set up the CSFB call, and wherein the fallback access network setting up the CSFB call comprises the fallback access network (i) setting up the CSFB call to the destination and (ii) paging the UE via the LTE access network to cause the UE to transition to be served by the fallback access network so as to engage in the CSFB call via the fallback access network.

11. The method of claim 8 , wherein signaling from the gateway to the MME to invoke setup of the CSFB call in place of the VoIP call comprises signaling from the gateway to the MME to cause the MME to signal to the UE to cause the UE to originate the CSFB call, and wherein the UE originating the CSFB call comprises the UE signaling via the LTE access network to a fallback access network to request origination of the CSFB call, and the UE transitioning to be served by the fallback access network so as to engage in the CSFB call via the fallback access network.

12. A network node through which voice over Internet Protocol (VoIP) call setup signaling passes between user equipment devices (UEs) and a VoIP call server, wherein the UEs are served by a first radio access network (RAN), the network node comprising: a network communication interface for sending and receiving the VoIP call setup signaling; a processing unit; non-transitory data storage; and program instructions stored in the non-transitory data storage and executable by the processing unit to carry out functions comprising (i) detecting failure of setup of a VoIP call from a UE to a call destination, wherein detecting the failure of setup of the VoIP call from the UE to the call destination is based on detecting failure to set up a dedicated VoIP bearer for the UE, wherein the network node is a gateway that provides connectivity between the first RAN and the packet-switched network, and wherein the dedicated VoIP bearer is between the UE and the gateway and (ii) responsive to the detecting failure of setup of the VoIP call, invoking setup of a circuit-switched fall back (CSFB) call between the UE and the call destination.

13. The network node of claim 12 , wherein invoking setup of the CSFB call between the UE and the call destination comprises signaling to a controller of the first RAN to cause the controller to invoke setup of the CSFB call.